tools: stricter no-undef eslint rule

By default eslint does not validate variables that are placed in a
typeof check.

PR-URL: nodejs#19926
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

.eslintrc.js

@@ -129,7 +129,7 @@ module.exports = {
     // Variables
     // http://eslint.org/docs/rules/#variables
     'no-delete-var': 'error',
-    'no-undef': 'error',
+    'no-undef': ['error', { typeof: true }],
     'no-unused-vars': ['error', { args: 'none' }],
     'no-use-before-define': ['error', {
       classes: true,